pytest: Mark Rust-dependent tests as skipped with VALGRIND

`valgrind` reports seems to flag some memory accesses that are ok in
the Rust standard library, which we can consider false positives for
our purposes:

```Valgrind error file: valgrind-errors.69147
==69147== Syscall param statx(file_name) points to unaddressable byte(s)
==69147==    at 0x4B049FE: statx (statx.c:29)
==69147==    by 0x2E2DA0: std::sys::unix::fs::try_statx (weak.rs:139)
==69147==    by 0x2D7BD5: <std::fs::File as std::io::Read>::read_to_string (fs.rs:784)
==69147==    by 0x2632CE: num_cpus::linux::Cgroup::param (linux.rs:214)
==69147==    by 0x263179: num_cpus::linux::Cgroup::quota_us (linux.rs:203)
==69147==    by 0x263002: num_cpus::linux::Cgroup::cpu_quota (linux.rs:188)
==69147==    by 0x262C01: num_cpus::linux::load_cgroups (linux.rs:149)
==69147==    by 0x26289D: num_cpus::linux::init_cgroups (linux.rs:129)
==69147==    by 0x26BD88: core::ops::function::FnOnce::call_once (function.rs:227)
==69147==    by 0x26B749: std::sync::once::Once::call_once::{{closure}} (once.rs:262)
==69147==    by 0x139717: std::sync::once::Once::call_inner (once.rs:419)
==69147==    by 0x26B6D5: std::sync::once::Once::call_once (once.rs:262)
==69147==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==69147==
==69147== Syscall param statx(buf) points to unaddressable byte(s)
==69147==    at 0x4B049FE: statx (statx.c:29)
==69147==    by 0x2E2DA0: std::sys::unix::fs::try_statx (weak.rs:139)
==69147==    by 0x2D7BD5: <std::fs::File as std::io::Read>::read_to_string (fs.rs:784)
==69147==    by 0x2632CE: num_cpus::linux::Cgroup::param (linux.rs:214)
==69147==    by 0x263179: num_cpus::linux::Cgroup::quota_us (linux.rs:203)
==69147==    by 0x263002: num_cpus::linux::Cgroup::cpu_quota (linux.rs:188)
==69147==    by 0x262C01: num_cpus::linux::load_cgroups (linux.rs:149)
==69147==    by 0x26289D: num_cpus::linux::init_cgroups (linux.rs:129)
==69147==    by 0x26BD88: core::ops::function::FnOnce::call_once (function.rs:227)
==69147==    by 0x26B749: std::sync::once::Once::call_once::{{closure}} (once.rs:262)
==69147==    by 0x139717: std::sync::once::Once::call_inner (once.rs:419)
==69147==    by 0x26B6D5: std::sync::once::Once::call_once (once.rs:262)
==69147==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==69147==

```

tests/test_cln_rs.py

@@ -5,11 +5,19 @@
 import pytest
 
 
-# Skip the entire module if we don't have Rust.
-pytestmark = pytest.mark.skipif(
-    env('RUST') != '1',
-    reason='RUST is not enabled, skipping rust-dependent tests'
-)
+# Skip the entire module if we don't have Rust. The same is true for
+# VALGRIND, since it sometimes causes false positives in
+# `std::sync::Once`
+pytestmark = [
+    pytest.mark.skipif(
+        env('RUST') != '1',
+        reason='RUST is not enabled skipping rust-dependent tests'
+    ),
+    pytest.mark.skipif(
+        env('VALGRIND') != '1',
+        reason='VALGRIND is enabled skipping rust-dependent tests, as they may report false positives.'
+    ),
+]
 
 
 def test_rpc_client(node_factory):